Recently, interest in energy storing technologies is gradually increasing. As the use of secondary battery expands into energies for a mobile phone, a camcorder, a laptop computer, and also to an electric vehicle, development of a chargeable and dischargeable secondary battery, or a lithium secondary battery in particular, is receiving increasing attentions.
Meanwhile, a porous separator of the secondary battery shows severe heat contraction behavior at a temperature of about 100° C. or above due to properties of the materials and characteristic of a fabricating process that involves elongation, thus resulting in a problem of short circuit generated between a positive electrode and a negative electrode.
The separator generally used in a lithium ion battery field may use a polyolefin-based material such as polyethylene or polypropylene, and has a thickness of about 25 μm.
Heat stability may be one of the issues raised with respect to development of the currently commercialized separator. The polymer resin separator starts to have heat contraction at a temperature of approximately 120 degrees. Specifically, a winding-type cylindrical/prismatic batteries have particularly greater deformation at a core portion where the stress is relatively weaker in T/D direction and thus are subject to possibilities that uncoated portions of the positive electrode and/or the negative electrode are contacted, heated, or ignited due to the separator contraction from exposure to a high temperature for a long time. In order to improve heat contraction of the separator, an inorganic-polymer resin composite may be prepared with a wet coating method. However, a thickness of the separator increases by more than several micrometers, causing deterioration of energy density of a battery cell in a limited space.
Further, when heats are locally generated and temperature rises to above certain degrees in the battery cell, the separator can be melt, thus causing short circuit between the positive electrode and the negative electrode, which in turn causes secondary heats to be generated, thus considerably deteriorating battery stability. Accordingly, constant demand is raised for a technology that can improve heat stability of the separator.